1. Field of the Invention
The present invention relates to a signal detecting circuit integrated into, for example, a general communication unit or an antitheft apparatus attached to an article, for reporting that a transmitted signal has been received.
2. Description of the Related Art
A conventional signal detecting circuit will be described below by referring to FIG. 15 with a signal detecting circuit for an antitheft apparatus attached to an article being taken as an example. Between the base and emitter of an npn transistor 51, a parallel resonant circuit 54 formed of a coil 52 and a capacitor 53 is connected. Therefore, the base and emitter of the transistor 51 have the same voltage. The resonant frequency of this parallel resonant circuit 54 is set such that it matches the frequency F of a signal to be received. The collector of the transistor 51 is connected to the base of a pnp transistor 56 through a load resistor 55. The emitter of the transistor 56 is connected to a power supply not shown through a terminal 57. The collector and emitter of the transistor 51 are connected to the collector and emitter of another npn transistor 58, respectively. A bias voltage is applied to the base of the transistor 58 through a diode 59 and a resistor 60 connected in series from the collector of the transistor 56. The base of the transistor 58 is also grounded through a time-constant circuit 63 formed of a capacitor 61 and a resistor 62 connected in parallel. Between the collector of the transistor 56 and the ground, alarm means 64 such as a buzzer is connected.
With the above configuration, when the parallel resonant circuit 54 receives, for example, a transmitted burst signal (hereinafter called just a signal) S at a specific frequency of F as shown in FIG. 16, the transistor 51 is turned on every positive half cycle because the base thereof is excited by the signal, and accordingly the transistor 56 is turned on. A voltage is intermittently applied to the collector of the transistor 56 from a power supply not shown. Therefore, a voltage is intermittently applied to the base of the transistor 58 through the diode 59 and the resistor 60. Since the signal S continues for a longer period (period T shown in FIG. 16) than the time constant of the time-constant circuit 63, the base voltage of the transistor 58 gradually rises during this period by a voltage with which the capacitor 61 of the time-constant circuit 63 is charged. The transistor 58 finally reaches a steady turn-on condition and the collector voltage thereof changes to almost 0 V. This change is fed back to the base of the transistor 56 and the base voltage of the transistor 56 decreases. As a result, the transistor 56 is also kept at the on state, a voltage from the power supply is applied to the alarm means 64, and it gives an alarm continuously.
This signal detecting circuit has been designed such that it is unlikely to malfunction against noise having a relatively short pulse width. When external noise having a shorter pulse width than the time constant of the time-constant circuit 63 is received, for example, even if the transistors 51 and 56 are temporarily turned on, since noise disappears while the base voltage of the transistor 58 has not yet sufficiently risen, the transistor 58 is not turned on. Therefore, the transistors 51 and 56 are turned off at a negative half cycle of the signal S and remain at the off state. A voltage is not applied to the alarm means 64 and an alarm is not given.
When the above conventional signal detecting circuit receives external noise having a shorter pulse width than the time constant of the time-constant circuit 63, even if the transistors 51 and 56 are temporarily turned on, the transistor 58 is not turned on due to the relationship between the pulse width of the external noise and the time constant of the time-constant circuit 63. Therefore, the transistor 56 is not continuously turned on. When noise having a longer pulse width than the time constant of the time-constant circuit 63 continues, however, the base voltage of the transistor 58 rises due to the time-constant circuit 63, and the transistor 58 is turned on. Once the transistor 58 is turned on, even if the external noise disappears, a voltage is continuously applied to the alarm means 64 and malfunction occurs.
To apply a voltage to the alarm means 64 when the transistor 58 is turned on, the base of the transistor 56 is grounded (the collector of the transistor 58 has almost 0 V when turned on) through the resistor 55. Therefore, a current continuously flows through a route formed of the transistor 56, the resistor 55, and the transistor 58 during an alarm or malfunction. The power consumed by the resistor 55 cannot be ignored. When a battery is used as a power source, this power consumption is a large problem.
This signal detecting circuit includes three transistors, 51, 56, and 58 and one diode 59. The number of components is not small and it increases the cost.
In the above conventional signal detecting circuit, the base and emitter of the transistor 51 have the same voltage due to the parallel resonant circuit 54 used for receiving a signal. Therefore, if a signal S input to the base does not exceed about 0.5 V, the transistor 51 is not turned on and receiving sensitivity is low.